Without being bound by any particular theory, the background of the present invention will be described by way of a description of particular problems discussed in the art and various proposed solutions to such problems. For brevity, various references will be briefly and generally summarized herein. A more complete understanding of such background art may be obtained by a complete review of the documents cited herein, etc. What should be understood from the following discussion is that, despite such extensive prior efforts to provide various methods of NOx removal and the like, a continuing need exists for practical and low-cost methods of NOx removal in a variety of industrial processes, which may utilize a variety of commercially-available reductants.
Carbonaceous fuels are burned in internal combustion engines and other equipment such as boilers, furnaces, heaters and incinerators, and the like (i.e., in a wide variety of industrial process). Excess air frequently is used to complete the oxidation of combustion byproducts such as carbon monoxide (CO), hydrocarbons and soot. High temperature combustion using excess air, however, tends to generate oxides of nitrogen (often referred to as NOx).
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO.sub.2). Free radicals of nitrogen (N.sub.2) and oxygen (O.sub.2) combine chemically primarily to form NO at high combustion temperatures. This thermal NOx tends to form even when nitrogen is removed from the fuel. Combustion modifications which decrease the formation of thermal NOx generally are limited by the generation of objectionable byproducts.
Mobile and stationary combustion equipment are concentrated sources of NOx emissions. When discharged to the air, emissions of NO oxidize to form NO.sub.2, which tends to accumulate excessively in many urban atmospheres. In sunlight, the NO.sub.2 reacts with volatile organic compounds to form groundlevel ozone, eye irritants and photochemical smog. These adverse effects have prompted extensive efforts for controlling NOx emissions to low levels. Despite advancements in fuel and combustion technology, groundlevel ozone concentrations still exceed federal guidelines in many urban regions. Under the Clean Air Act and its amendments, these ozone nonattainment areas must implement stringent NOx emissions regulations. Such regulations will require low NOx emissions levels that are attained only by exhaust aftertreatment.
Exhaust aftertreatment techniques tend to reduce NOx using various chemical or catalytic methods. Such methods are known in the art and involve nonselective catalytic reduction (NSCR), selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Alternatively, NO may be oxidized to NO.sub.2 for removal by wet scrubbers. Such aftertreatment methods typically require some type of reactant for removal of NOx emissions.
Wet scrubbing of NO.sub.2 produces waste solutions that represent potential sources of water pollution. Wet scrubbers primarily are used for NOx emissions from nitric acid plants or for concurrent removal of NO.sub.2 with sulfur dioxide (SO.sub.2). High costs and complexity generally limit scrubber technology to such special applications. Wet scrubbers are applied to combustion exhaust by converting NO to NO.sub.2, such as is described in U.S. Pat. No. 5,047,219.
The NSCR method typically uses unburned hydrocarbons and CO to reduce NOx emissions in the absence of O.sub.2. Fuel/air ratios must be controlled carefully to ensure low excess O.sub.2. Both reduction and oxidation catalysts are needed to remove emissions of CO and hydrocarbons while also reducing NOx. The cost of removing excess O.sub.2 precludes practical applications of NSCR methods to many O.sub.2 -containing exhaust gases.
Combustion exhaust containing excess O.sub.2 generally requires chemical reductant(s) for NOx removal. Commercial SCR systems primarily use ammonia (NH.sub.3) as the reductant. Chemical reactions on a solid catalyst surface convert NOx to N.sub.2. These solid catalysts are selective for NOx removal and do not reduce emissions of CO and unburned hydrocarbons. Excess NH.sub.3 needed to achieve low NOx levels tends to result in NH.sub.3 breakthrough as a byproduct emission.
Large catalyst volumes are normally needed to maintain low levels of NOx and NH.sub.3 breakthrough. The catalyst activity depends on temperature and declines with use. Normal variations in catalyst activity are accommodated only by enlarging the volume of catalyst or limiting the range of combustion operation. Catalysts may require replacement prematurely due to sintering or poisoning when exposed to high levels of temperature or exhaust contaminants. Even under normal operating conditions, the SCR method requires a uniform distribution of NH.sub.3 relative to NOx in the exhaust gas. NOx emissions, however, are frequently distributed nonuniformly, so low levels of both NOx and NH.sub.3 breakthrough may be achieved only by controlling the distribution of injected NH.sub.3 or mixing the exhaust to a uniform NOx level.
NH.sub.3 breakthrough is alternatively limited by decomposing excess NH.sub.3 on the surface of a catalyst as described in U.S. Pat. No. 4,302,431. In this case, the excess NH.sub.3 is decomposed catalytically following an initially equivalent decomposition of NOx and NH.sub.3 together. The decomposition of excess NH.sub.3, however, reduces the selectivity of the SCR method, increasing the molar ratio of NH.sub.3 with respect to NOx as much as 1.5 or higher.
In a combination of catalytic and noncatalytic reduction methods, both NOx and NH.sub.3 removal may be controlled by SCR following an initial stage of NOx reduction by SNCR. In the SNCR method, NOx emissions may be reduced partially without controlling NH.sub.3 breakthrough to a low level. The SCR method may decrease NOx further while also lowering NH.sub.3 breakthrough to an acceptable level.
The use of excess NH.sub.3 to enhance NOx removal by the SNCR method is described in detail in U.S. Pat. Nos. 4,978,514 and 5,139,754. With such methods, the NH.sub.3 injection to SNCR is controlled so that the unreacted NH.sub.3 remains sufficient for the subsequent catalytic reduction of NOx to a low level. This injection strategy is based on the use of excess NH.sub.3 for reducing NOx to lower levels, as with the SCR method described above.
Another method for combining SNCR and SCR methods is described in U.S. Pat. No. 5,510,092. In this method, the catalytic NOx reduction is always maximized using a separate NH.sub.3 injection grid, and the NOx emissions are reduced noncatalytically only as needed to maintain a final low NOx level. This method decreases the consumption of NH.sub.3 by minimizing the use of SNCR which removes NOx less selectively than the catalytic method.
The low selectivity of the SNCR method and the use of excess NH.sub.3 for decreasing NOx levels is reported by Lyon, who is believed to have first suggested the noncatalylic reduction of NOx (U.S. Pat. No. 3,900,554). In commercial coal-fired boiler tests, 73% NO reduction has been reported with 2.2 ppm NH.sub.3 breakthrough using a 0.9 molar ratio of NH.sub.3 with respect to NO, while 86% NO reduction required 11 ppm NH.sub.3 breakthrough and a 2.2 molar ratio of NH.sub.3 with respect to NOx. These results are reported in Environ. Sci. Technol., Vol. 21, No. 3, pages 231-236 1987.
In another article (Ind. Eng. Chem. Fundam., Vol. 25, No. 1, pages 19-24 1986), Lyon also reports the inhibiting effect of NH.sub.3 on CO oxidation. This observation in experiments and commercial tests is confirmed by modeling studies. The inhibition has been explained in terms of competition between NH.sub.3 and CO for reaction with the OH free radical. It is believed that, while NH.sub.3 inhibits the oxidation of CO, the CO also decreases the selectivity of NO reduction by NH.sub.3.
This "sacrifice of residual CO oxidation" is described by Lyon as an important limitation of the noncatalytic reduction method. According to these teachings, the injection of NH.sub.3 should follow the completion of CO oxidation in order to overcome this limitation. When NH.sub.3 is injected before the completion of CO oxidation, the oxidation of residual CO tends to diminish, which may result in greater levels of byproduct CO emissions.
Despite this disadvantage of greater byproduct CO emissions, many patents teach the use of CO or other ancillary reducing materials to lower the effective temperature for reducing NO noncatalytically. For example, the use of CO to lower the temperature for NO reduction by HNCO is discussed in U.S. Pat. No. 4,886,650 as follows: "Where it is desired to lower the operating temperature to a greater degree, larger amounts of CO or other H atom generating species will be added or vice-versa."
This lowering of the effective temperature for NO reduction has been repeated generally in a consistent manner throughout patent literature related to the noncatalytic method. The original discoveries of NH.sub.3 (U.S. Pat. No. 3,900,554) and urea (U.S. Pat. No. 4,208,386) as NOx reductants in the temperature range of 1600 to 2000.degree. F. both reported the use of ancillary reducing materials to enable noncatalytic NO reduction throughout the temperature range of 1300 to 2000.degree. F. Hydrogen (H.sub.2), CO, and hydrocarbon(s), including oxygenated hydrocarbons, have been proposed as ancillary reducing materials that may lower the effective temperature for noncatalytic NO reduction by NH.sub.3 or urea. This use of hydrocarbon(s) and CO is reportedly limited, however, due to incomplete oxidation, resulting in the production of air pollutants. Hydrogen has been cited as the preferred reducing material because it does not produce any air pollutants.
The use of hydrogen is limited because it decreases the selectivity for NO reduction by NH.sub.3 or urea. To overcome this limitation, the hydrogen may be added in successive multiple stages as described in U.S. Pat. No. 3,900,554. A more detailed description of a multi-stage method for noncatalytic NO reduction using NH.sub.3 and hydrogen is disclosed in U.S. Pat. No. 4,115,515. This multi-stage method typically requires two or more locations along the flowpath of the exhaust gas to inject reducing gas mixtures. The optimum use of multiple injection stages and alternative reducing gas mixtures depends on the exhaust gas temperature in the vicinity of each injection location. The multi-stage method accounts for temperature gradients along the gas flowpath as well as variations in temperature at each injection location.
Generally according to the these teachings, NH.sub.3 is injected as the only reducing gas at temperatures in the range of 900 to 1000.degree. C. (about 1650 to 1850.degree. F.), while mixtures of NH.sub.3 and hydrogen are injected at temperatures in the range of 700 to 900.degree. C. (about 1300 to 1650.degree. F.). Decreasing the NO concentration in the first injection stage using NH.sub.3 alone minimizes the less-selective reduction of NO in the second stage where hydrogen is used as the ancillary reducing material.
Patents have proliferated since such disclosures of noncatalytic reduction methods. In particular, U.S. Pat. Nos. 4,731,231, 4,800,068, 4,886,650 and 4,908,193 have disclosed the decomposition of cyanuric acid, (HNCO).sub.3, to generate isocyanic acid (HNCO) for NO reduction. Also, other patents (for example, U.S. Pat. Nos. 4,719,092, 4,751,065, 4,770,863, 4,803,059, 4,844,878, 4,863,705, 4,873,066, 4,877,591, 4,888,165, 4,927,612 and 4,997,631) have disclosed a variety of reducing materials as alternatives to NH.sub.3, urea or cyanuric acid, or as enhancers for use with NH.sub.3, urea or cyanuric acid.
Such patents primarily address the acute problems of NH.sub.3 breakthrough and byproduct CO emissions that are characteristic of the noncatalytic reduction method. In addition to the disclosures of various reductants and enhancers, other patents (for example, U.S. Pat. Nos. 4,777,024, 4,780,289, 4,863,704, 4,877,590, 4,902,488, 4,985,218, 5,017,347 and 5,057,293) describe elaborate control strategies and multi-stage injection methods.
Such control strategies and multi-stage methods primarily address variations in temperature. Combustion equipment typically operate throughout a load range, and exhaust gas temperatures generally increase at higher loads. Therefore, local temperatures vary at the fixed locations where the reductant(s) and reducing material(s) are injected into the exhaust gas. The noncatalytic methods do not control the local temperature for NOx reduction.
With noncatalytic reduction methods, the local temperature typically is used as a means for controlling the injection of reductant(s) and ancillary reducing material(s). The patents teach the use of ancillary reducing material(s) to lower the effective temperature for NOx reduction so that it matches the actual local temperature, which depends solely on the production of the exhaust gas. It is important to note that, in such teachings, the ancillary reducing material(s) are not injected to control the local temperature. Ancillary reducing material(s) mall enable NOx reduction at lower effective temperatures, but may result in the formation of objectionable byproducts. Such teachings tend only to minimize disadvantages of noncatalytic reduction methods. Such techniques in general do not provide for concurrent depletion of NH.sub.3 and CO emissions.
The use of oxygenated hydrocarbons is described in U.S. Pat. No. 4,830,839 as a means for scrubbing NH.sub.3 breakthrough from a previous stage of noncatalytic NOx reduction. With this method, vaporized concentrations of oxygenated hydrocarbons in the range of 2 to 500 ppm are added to the exhaust gas so that their weight ratios with respect to NH.sub.3 are in the range of 2 to 200. U.S. Pat. No. 5,047,219, however, subsequently discloses that oxygenated hydrocarbons oxidize NO to NO.sub.2 at temperatures below about 1600.degree. F.
Lowering the effective temperature for noncatalytic reduction below about 1700.degree. F. also slows the thermal decomposition of nitrous oxide (N.sub.2 O) as described in U.S. Pat. No. 5,048,432. This patent teaches reheating of exhaust gases using a burner with a separate source of combustion air. The N.sub.2 O is decomposed thermally when the high temperature flue gases from the burner mix and reheat the primary exhaust gas above 1700.degree. F.
Formation of N.sub.2 O as a noncatalytic reduction byproduct is described in U.S. Pat. No. 4,997,631. When NOx emissions are reduced by the noncatalytic method, a portion of the reduced NOx is converted to N.sub.2 O. As described above, N.sub.2 O levels decrease at higher temperatures, but also the reported data suggest much less N.sub.2 O formation when NH.sub.3 is used as the chemical reductant. Urea and cyanuric acid reportedly result in higher N.sub.2 O levels.
A different method for staging noncatalytic NO reduction is described in U.S. Pat. No. 3,867,507. Hydrocarbons are disclosed to reduce NO when the molar ratio of O.sub.2 with respect to carbon is less than 2.5. Such low O.sub.2 levels, however, tend to result in the formation of objectionable byproducts, including NH.sub.3, hydrogen cyanide (HCN), CO and unburned hydrocarbons. Such byproducts are removed by oxidation using added air at elevated temperatures, e.g., 1100.degree. C. (about 2000.degree. F.), in a second stage.
Similar methods for staging noncatalytic NOx reduction are disclosed in U.S. Pat. Nos. 4,851,201 and 4,861,567. With these methods, reductant(s) are mixed with the exhaust gas and decomposed under fuel-rich combustion conditions in a first stage, and then NOx is reduced in a second stage with an excess of oxygen. The temperature and oxygen concentration are adjusted between the two stages. The temperature ranges for each stage depend on whether the reductant is cyanuric acid rather than NH.sub.3 or urea.
Another method for lowering the effective temperature for noncatalytic NOx reduction is described in Int. App. No. PCT/US92/07212 (Pub. No. WO 93/03998). It is suggested that hydrocarbons be injected so as to create stratified mixtures effective for generating partial oxidation products as ancillary reducing materials to lower the effective temperature for NOx reduction using cyanuric acid.
Based on such previous teachings, noncatalytic NOx reduction by hydrocarbon(s) alone tends to be limited to fuel-rich combustion conditions, i.e., low O.sub.2 levels. In the presence of excess O.sub.2, hydrocarbon(s) and CO are ineffective for NOx reduction, but may be used to lower the effective temperature for noncatalytic NOx reduction using chemical reductint(s). In such an ancillary role, such materials also are claimed to lower NH.sub.3 breakthrough, but such would be achieved only at the expense of decreased selectivity for NOx reduction.
Under these conditions, noncatalytic NOx reduction tends to be limited because excess NH.sub.3 breakthrough is needed to achieve low NOx levels, but the oxidation of CO is inhibited by NH.sub.3, so the lowering of noncatalytic reduction temperature using ancillary materials tends to increase byproduct CO emissions. Both NH.sub.3 and CO are objectionable SNCR byproducts. In general, even the most elaborate proposed SNCR controls and staging methods cannot deplete these objectionable byproducts concurrently. In general, such methods only minimize the disadvantageous production of one byproduct at the expense of increasing another.
Furthermore, noncatalytic reduction methods are highly dependent upon temperature, but tend to provide no means for controlling this key condition. Staging methods and elaborate controls are needed to maintain an effective temperature for the chemical reductint(s) when excess O.sub.2 is present at the local exhaust gas conditions. Injecting reductant(s) under specific fuel-rich combustion conditions also is claimed to require a staged introduction of excess air to complete the combustion of primary fuel.
Despite such staging methods and elaborate controls, the NH.sub.3 breakthrough from SNCR in general is depleted only by using a subsequent catalytic method. Such SCR methods, however, do not remove the objectionable byproduct CO emissions from SNCR without using a separate oxidation catalyst similar to NSCR. Alternatively, this disadvantage of SNCR may be minimized to the greatest extent by limiting noncatalytic reduction only to maintain a final low NOx level in combination with SCR as described in U.S. Pat. No. 5,510,092.
Relegating the inherent advantages of a gas-phase method for reducing NOx to a subordinate role, however, does not minimize the key disadvantages of using solid catalysts. The expensive installation of large catalyst volumes intrudes adversely upon the combustion equipment. The catalyst bed adds pressure drop, and the vaporization of NH.sub.3 may derate the combustion equipment by as much as 2%.
As is well known, solid catalysts tend to gradually become plugged and poisoned under normal use and require periodic replacement. Premature replacement is needed when the catalyst is sintered or poisoned due to unusually high temperatures or contaminant levels resulting from combustion-related problems or other equipment failures. Like SNCR methods, catalytic reduction methods are highly dependent upon on temperature, but provide no means for controlling this key condition.
In considering the foregoing teachings and efforts, and in particular in view of the continuing need in numerous applications for practical, cost-effective reduction of NOx despite the extensive previous efforts, a need exists for new methods for selective NOx reduction, which may in general combine certain advantages of previously disclosed methods substantially, but without disadvantages thereof.
As will be described hereinafter, Applicants submit that they have discovered such methods.
It should be understood that the foregoing discussion of the background of the present invention, and the detailed description of the present invention to follow, is provided for understanding the context and applicability of the present invention, and is provided without being bound by any particular theory or the like. References to particular background patents or other materials are for general discussion purposes only and based on Applicants' understanding thereof, and the complete references should be consulted for the actual contents of such patents and materials.